The addition of sensitizing dye(s) to a silver halide emulsion to enlarge the sensitive wavelength range of the emulsion so as to optically sensitize is well known in the field of manufacturing silver halide photographic materials.
Many color sensitizing dyes for this purpose have heretofore been known. For instance, they include cyanine dyes, merocyanine dyes, xanthene dyes and others as described in T. H. James, The Theory of the Photographic Process, 3rd Ed., pages 198 to 228 (published by Macmillan Co., N.Y., 1966).
When such sensitizing dyes are applied to silver halide emulsions, in general, they must not merely enlarge the sensitive wavelength range of the silver halide emulsions, they must also satisfy the following conditions:
(1) they have a suitable color-sensitizing range; PA1 (2) they have a high sensitizing efficiency and may yield a sufficiently high sensitivity; PA1 (3) they do not cause fogging; PA1 (4) they may sensitize silver halide emulsions so that the sensitivity of the sensitized emulsion does not fluctuate much under variation of the ambient temperature in exposure; PA1 (5) they do not yield any bad interaction with other additives, such as a stabilizer, an antifoggant, a coating aid and a coupler; PA1 (6) when silver halide emulsions to which sensitizing dyes have been added are stored, the sensitivity of the emulsions does not fluctuate; in particular, when they are stored under high temperature and humidity conditions, they are free from fluctuation of sensitivity thereof; and PA1 (7) the sensitizing dyes as added to silver halide emulsions do not diffuse to any other light-sensitive layers to cause color mixing after development.
The above-mentioned conditions are important in preparing silver halide emulsions for silver halide photographic materials. In particular, elevation of the sensitivity of silver halides and improvement of the stability of raw films during storage, satisfying in the above conditions in the (2) and (6) are strongly desired.
Production of cyanine dyes and merocyanine dyes for use in the present invention, which have a crosslinked structure in the methine chain moiety, is described in A. I. Tolmachev, Yu. L. Slominskii, and Academician A. I. Kiprianov, Dokl. Akad. Nauk SSSR, Vol. 177, page 869 (1967). Application of only cyanine dyes to silver halide photographic materials is described in U.S. Pat. Nos. 3,582,344 and 5,013,642 and European Patents 420,011 and 432,473, but use of the merocyanine dyes of the present invention has heretofore been unknown.